When someone dies a ‘violent or unnatural death’ or a ‘sudden death of which the cause is unknown’, the death has to be reported to the coroner. An inquest must be held to identify the person and answer the questions ‘how, when and where’ the person died, and if there is to be criminal proceedings. Given that drug deaths usually occur from either accidental or purposeful poisoning (overdose), they are often sudden. This is particularly difficult because there is no time for families and friends to prepare, where they can think about the death or cultivate skills and attitudes that might help them cope after the bereavement. 
Another factor which might come into play for families is actually discovering the existence or true scale of their loved one’s substance use, and some family members may have no idea that the person used drugs at all. 
If you are concerned about the circumstances of a death, seek legal advice from a specialist solicitor.